1. Field of the Invention
The invention relates to an implantable cardiac stimulator for treatment of tachycardiac arrhythmias. Such a cardiac stimulator is also known as an implantable cardioverter/defibrillator (ICD) and usually also fulfills the function of an implantable cardiac pacemaker.
2. Description of the Related Art
The implantable cardiac stimulator discussed herein comprises:                a chamber stimulation unit, which is connected or connectable to a left ventricular or right ventricular stimulation electrode and is designed to generate and deliver chamber stimulation pulses for stimulation of a ventricle of a heart,        a ventricular sensing unit, which is designed to detect a respective chamber contraction and to deliver a ventricular sensing signal in the case of a detected chamber contraction,        an atrial sensing unit, which is designed to detect an atrial contraction, and in the case when an atrial contraction is detected, to deliver a respective atrial sensing signal indicating a respective atrial event,        a tachycardia detection unit which is connected to at least one ventricular sensing unit and is designed to detect a tachycardia and to categorize it as a ventricular tachycardia (VT) or as a supraventricular tachycardia (SVT), and        a treatment control unit, which is designed to control at least the chamber stimulation unit for delivering antitachycardiac stimulation (ATP).        
Stimulation and sensing electrodes may be identical to one another and may alternately serve to deliver stimulation pulses and receive myocardial potentials.
Tachyarrhythmia is understood to be an arrhythmia which leads to a heart rate higher than is physiologically appropriate. A distinction is made between tachycardia and fibrillations. In tachycardia (cardiac flutter), there is still a contraction of the affected chamber of the heart, but fibrillation is characterized in that the affected chamber of the heart is exposed to a recurring stimulation in which parts of the myocardium of the heart chamber are relaxing already while other parts are just beginning to contract, so there is no periodic contraction and relaxation of the heart chamber with the volume change that is required for pumping. Of the tachycardias, a distinction is made between ventricular tachycardias (VT) and supraventricular tachycardias (SVT), which are tachyarrhythmias of one or both heart chamber (right and/or left ventricles). Whereas supraventricular tachycardias have their origin in the atrium and are conducted to the respective ventricle by atrioventricular stimulus conduction, ventricular tachycardias have their origin in the respective ventricle itself.
Currently all manufacturers of implantable cardioverter defibrillators (ICDs) offer devices capable of discriminating between ventricular (VT) and supraventricular tachycardia therapy (SVT). One thing these instruments all have in common is that they ascertain the origin of the tachyarrhythmia before treating any tachyarrhythmia and, depending on its origin, initiate a corresponding treatment.
However, known approaches for differentiating between ventricular and supraventricular tachycardia consist of classifying irregular rapid atrial and ventricular rhythms as VT, for example, and classifying regular rapid rhythms as SVT; see, for example, U.S. Pat. No. 5,686,793, U.S. Pat. No. 5,891,170, U.S. Pat. No. 6,748,269, U.S. Pat. No. 6,889,080, U.S. Pat. No. 7,174,209 and US 2004/0093037. It is possible in this way to discriminate between VT and SVT even before initiating a corresponding therapy.
None of the instruments available currently has 100% specificity, i.e., this results in inadequate delivery of treatment. In this context, painful delivery of shocks should be regarded as especially critical because they would drastically lower acceptance of ICD therapy.
So-called “active” discrimination algorithms capable of improving the discrimination result by means of targeted ventricular or atrial stimulation and observation of the next respective chamber (atrium or ventricle) have not become established. The main disadvantage here is the delay in the detection decision associated with the stimulation phase and the proarrhythmic potential of such stimulation prior to the detection decision.